236 
PROFESSOR J. A. EWIRG AXD MR. W. LOW OX THE 
other cases; the use of the more permeable metal—wrought iron—for the cones has, of 
course, the advantag-e of strengthening’ the induction in the neck. Here the residual 
magnetism was measured, after the observations were otherwise complete, by slackening- 
one of the cones, so that it might be slipped oft the spindle. A suitable induction 
coil, wound on a ring, was then slipped on ; the whole bobbin was magnetised and 
removed from the field, the loose end and the coil were then slipped off together, and 
the ballistic effect of this was observed. In these measurements the bobbin was 
demagnetised by the method of reversals, to get rid of the effect of previous stronger 
magnetisations. A similar procedure was followed in finding the residual magnetism 
of steel samples. The residual magnetism is allowed for in the values of ^13 given 
below. 
Fig. 12. 
Table VIII.—Cast Iron. 
Current in field 
magnets, ampferes. 
Outside field. 
SB — outside field 
SB 
4- 
outside field 
1-57 
4,560 
20,070 
1230 
4-40 
3-62 
9,120 
24,630 
1230 
2-70 
5-95 
11,770 
27,680 
1270 
2-35 
8T 
13,460 
28,710 
1210 
2T3 
14-2 
14,690 
30,160 
1230 
2-05 
23-0 
16,200 
30,920 
1170 
1-91 
40-0 
16,900 
31,760 
1180 
1-88 
These two experiments agree in assigning al^out 1240 as the saturation value of 3 
in this cast iron; and the apparent diminution in fields of the greatest strength is, of 
course, to be set down to the cause which has been fully explained in connection with 
wrought iron—an excess of the “ outside field ” over the mean force within the metal, 
owing to the cones being too blunt to give a veiy uniform field. 
